Trends 2-1 2. Trends in Greenhouse Gas Emissions 2.1 Recent Trends in U.S. Greenhouse Gas Emissions and Sinks In 2015, total gross U.S. greenhouse gas emissions were 6,586.7 MMT, or million metric tons, carbon dioxide (CO 2 ) Eq. Total U.S. emissions have increased by 3.5 percent from 1990 to 2015, and emissions decreased from 2014 to 2015 by 2.3 percent (153.0 MMT CO 2 Eq.). The decrease in total greenhouse gas emissions between 2014 and 2015 was driven in large part by a decrease in CO 2 emissions from fossil fuel combustion. The decrease in CO 2 emissions from fossil fuel combustion was a result of multiple factors, including: (1) substitution from coal to natural gas consumption in the electric power sector; (2) warmer winter conditions in 2015 resulting in a decreased demand for heating fuel in the residential and commercial sectors; and (3) a slight decrease in electricity demand. Since 1990, U.S. emissions have increased at an average annual rate of 0.2 percent. Figure 2-1 through Figure 2-3 illustrate the overall trend in total U.S. emissions by gas, annual changes, and absolute changes since 1990. Overall, net emissions in 2015 were 11.5 percent below 2005 levels as shown in Table 2-1. Figure 2-1: Gross U.S. Greenhouse Gas Emissions by Gas (MMT CO2 Eq.)
36
Embed
2. Trends in Greenhouse Gas Emissions...Trends 2-1 2. Trends in Greenhouse Gas Emissions 2.1 Recent Trends in U.S. Greenhouse Gas Emissions and Sinks In 2015, total gross U.S. greenhouse
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Trends 2-1
2. Trends in Greenhouse Gas Emissions
2.1 Recent Trends in U.S. Greenhouse Gas Emissions and Sinks
In 2015, total gross U.S. greenhouse gas emissions were 6,586.7 MMT, or million metric tons, carbon dioxide (CO2)
Eq. Total U.S. emissions have increased by 3.5 percent from 1990 to 2015, and emissions decreased from 2014 to
2015 by 2.3 percent (153.0 MMT CO2 Eq.). The decrease in total greenhouse gas emissions between 2014 and 2015
was driven in large part by a decrease in CO2 emissions from fossil fuel combustion. The decrease in CO2 emissions
from fossil fuel combustion was a result of multiple factors, including: (1) substitution from coal to natural gas
consumption in the electric power sector; (2) warmer winter conditions in 2015 resulting in a decreased demand for
heating fuel in the residential and commercial sectors; and (3) a slight decrease in electricity demand. Since 1990,
U.S. emissions have increased at an average annual rate of 0.2 percent. Figure 2-1 through Figure 2-3 illustrate the
overall trend in total U.S. emissions by gas, annual changes, and absolute changes since 1990. Overall, net emissions
in 2015 were 11.5 percent below 2005 levels as shown in Table 2-1.
Figure 2-1: Gross U.S. Greenhouse Gas Emissions by Gas (MMT CO2 Eq.)
2-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2015
Figure 2-2: Annual Percent Change in Gross U.S. Greenhouse Gas Emissions Relative to the Previous Year
Figure 2-3: Cumulative Change in Annual Gross U.S. Greenhouse Gas Emissions Relative to 1990 (1990=0, MMT CO2 Eq.)
Overall, from 1990 to 2015, total emissions of CO2 increased by 288.4 MMT CO2 Eq. (5.6 percent), while total
emissions of methane (CH4) decreased by 125.1 MMT CO2 Eq. (16.0 percent), and total emissions of nitrous oxide
(N2O) decreased by 24.7 MMT CO2 Eq. (6.9 percent). During the same period, aggregate weighted emissions of
hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6), and nitrogen trifluoride (NF3) rose
by 85.0 MMT CO2 Eq. (85.3 percent). Despite being emitted in smaller quantities relative to the other principal
greenhouse gases, emissions of HFCs, PFCs, SF6, and NF3 are significant because many of them have extremely
high global warming potentials (GWPs), and, in the cases of PFCs, SF6, and NF3, long atmospheric lifetimes.
Conversely, U.S. greenhouse gas emissions were partly offset by carbon (C) sequestration in managed forests, trees
in urban areas, agricultural soils, landfilled yard trimmings, and coastal wetlands. These were estimated to offset
11.8 percent of total emissions in 2015.
As the largest contributor to U.S. greenhouse gas emissions, CO2 from fossil fuel combustion has accounted for
approximately 77 percent of GWP-weighted emissions for the entire time series since 1990. Emissions from this
source category grew by 6.5 percent (309.4 MMT CO2 Eq.) from 1990 to 2015 and were responsible for most of the
increase in national emissions during this period. In addition, CO2 emissions from fossil fuel combustion decreased
Trends 2-3
from 2005 levels by 697.2 MMT CO2 Eq., a decrease of approximately 12.1 percent between 2005 and 2015. From
2014 to 2015, these emissions decreased by 2.9 percent (152.5 MMT CO2 Eq.). Historically, changes in emissions
from fossil fuel combustion have been the dominant factor affecting U.S. emission trends.
Changes in CO2 emissions from fossil fuel combustion are influenced by many long-term and short-term factors,
including population and economic growth, energy price fluctuations and market trends, technological changes,
energy fuel choices, and seasonal temperatures. On an annual basis, the overall consumption and mix of fossil fuels
in the United States fluctuates primarily in response to changes in general economic conditions, overall energy
prices, the relative price of different fuels, weather, and the availability of non-fossil alternatives. For example, coal
consumption for electricity generation is influenced by a number of factors including the relative price of coal and
alternative sources, the ability to switch fuels, and longer terms trends in coal markets. Likewise, warmer winters
will lead to a decrease in heating degree days and result in a decreased demand for heating fuel and electricity for
heat in the residential and commercial sector, which leads to a decrease in emissions from reduced fuel use.
Energy-related CO2 emissions also depend on the type of fuel or energy consumed and its C intensity. Producing a
unit of heat or electricity using natural gas instead of coal, for example, can reduce the CO2 emissions because of the
lower C content of natural gas (see Table A-39 in Annex 2.1 for more detail on the C Content Coefficient of
different fossil fuels).
A brief discussion of the year to year variability in fuel combustion emissions is provided below, beginning with
2011.
Recent trends in CO2 emissions from fossil fuel combustion show a 3.9 percent decrease from 2011 to 2012, then a
2.6 percent and a 0.9 percent increase from 2012 to 2013 and 2013 to 2014, respectively, and a 2.9 percent decrease
from 2014 to 2015. Total electricity generation remained relatively flat over that time period but emission trends
generally mirror the trends in the amount of coal used to generate electricity. The consumption of coal used to
generate electricity decreased by roughly 12 percent from 2011 to 2012, increased by 4 percent from 2012 to 2013,
stayed relatively flat from 2013 to 2014, and decreased by 14 percent from 2014 to 2015. The overall CO2 emission
trends from fossil fuel combustion also follow closely changes in heating degree days over that time period. Heating
degree days decreased by 13 percent from 2011 to 2012, increased by 18 percent from 2012 to 2013, increased by 2
percent from 2013 to 2014, and decreased by 10 percent from 2014 to 2015. The overall CO2 emission trends from
fossil fuel combustion also generally follow changes in overall petroleum use and emissions. Carbon dioxide
emissions from petroleum decreased by 2.0 percent from 2011 to 2012, increased by 1.6 percent from 2012 to 2013,
increased by 0.8 percent from 2013 to 2014, and increased by 1.7 percent from 2014 to 2015. The increase in
petroleum CO2 emissions from 2014 to 2015 somewhat offset emission reductions from decreased coal use in the
electricity sector from 2014 to 2015.
Table 2-1 summarizes emissions and sinks from all U.S. anthropogenic sources in weighted units of MMT CO2 Eq.,
while unweighted gas emissions and sinks in kilotons (kt) are provided in Table 2-2.
Table 2-1: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (MMT CO2 Eq.)
Net Emissions (Sources and Sinks) 5,543.5 6,582.3 6,027.6 5,784.5 5,917.1 5,978.3 5,827.7
Notes: Total emissions presented without LULUCF. Net emissions presented with LULUCF. Totals may not sum
due to independent rounding. Parentheses indicate negative values or sequestration.
+ Does not exceed 0.05 MMT CO2 Eq. a There was a method update in this Inventory for estimating the share of gasoline used in on-road and non-road
applications. The change does not impact total U.S. gasoline consumption. It mainly results in a shift in gasoline
consumption from the transportation sector to industrial and commercial sectors for 2015, creating a break in the
time series. The change is discussed further in the Planned Improvements section of Chapter 3.1. b Emissions from Wood Biomass, Ethanol, and Biodiesel Consumption are not included specifically in summing
Energy sector totals. Net carbon fluxes from changes in biogenic carbon reservoirs are accounted for in the
estimates for LULUCF. c Emissions from International Bunker Fuels are not included in totals. d Small amounts of PFC emissions also result from this source. e LULUCF emissions include the CH4 and N2O emissions reported for Peatlands Remaining Peatlands, Forest Fires,
Drained Organic Soils, Grassland Fires, and Coastal Wetlands Remaining Coastal Wetlands; CH4 emissions from
Land Converted to Coastal Wetlands; and N2O emissions from Forest Soils and Settlement Soils. f LULUCF Carbon Stock Change is the net C stock change from the following categories: Forest Land Remaining
Forest Land, Land Converted to Forest Land, Cropland Remaining Cropland, Land Converted to Cropland,
Grassland Remaining Grassland, Land Converted to Grassland, Wetlands Remaining Wetlands, Land Converted to
Wetlands, Settlements Remaining Settlements, and Land Converted to Settlements. Refer to Table 2-8 for a
breakout of emissions and removals for LULUCF by gas and source category. g The LULUCF Sector Net Total is the net sum of all CH4 and N2O emissions to the atmosphere plus net carbon
stock changes.
Table 2-2: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (kt)
M - Mixture of multiple gases a There was a method update in this Inventory for estimating the share of gasoline used in on-road and non-road applications. The
change does not impact total U.S. gasoline consumption. It mainly results in a shift in gasoline consumption from the
transportation sector to industrial and commercial sectors for 2015, creating a break in the time series. The change is discussed
further in the Planned Improvements section of Chapter 3.1.
b Emissions from Wood Biomass, Ethanol, and Biodiesel Consumption are not included specifically in summing Energy sector
totals. Net carbon fluxes from changes in biogenic carbon reservoirs are accounted for in the estimates for LULUCF. c Emissions from International Bunker Fuels are not included in totals. d Small amounts of PFC emissions also result from this source.
Notes: Totals may not sum due to independent rounding. Parentheses indicate negative values or sequestration.
Emissions of all gases can be summed from each source category into a set of five sectors defined by the
Intergovernmental Panel on Climate Change (IPCC). Figure 2-4 and Table 2-3 illustrate that over the twenty-six
year period of 1990 to 2015, total emissions in the Energy, Industrial Processes and Product Use, and Agriculture
sectors grew by 221.0 MMT CO2 Eq. (4.1 percent), 35.5 MMT CO2 Eq. (10.4 percent), and 27.0 MMT CO2 Eq. (5.5
percent), respectively. Emissions from the Waste sector decreased by 59.9 MMT CO2 Eq. (30.1 percent). Over the
same period, estimates of net C sequestration for the Land Use, Land-Use Change, and Forestry sector (magnitude
2-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2015
of emissions plus CO2 removals from all LULUCF categories) increased by 60.7 MMT CO2 Eq. (7.4 percent
decrease in net C sequestration).
Figure 2-4: U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector (MMT CO2 Eq.)
Table 2-3: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector (MMT CO2 Eq.)
Net Emission (Sources and Sinks)c 5,543.5 6,582.3 6,027.6 5,784.5 5,917.1 5,978.3 5,827.7
Notes: Total emissions presented without LULUCF. Net emissions presented with LULUCF. a There was a method update in this Inventory for estimating the share of gasoline used in on-road and non-road applications.
The change does not impact total U.S. gasoline consumption. It mainly results in a shift in gasoline consumption from the
transportation sector to industrial and commercial sectors for 2015, creating a break in the time series. The change is
discussed further in the Planned Improvements section of Chapter 3.1. b Total emissions without LULUCF. c Net emissions with LULUCF.
Notes: Totals may not sum due to independent rounding. Parentheses indicate negative values or sequestration.
Energy Energy-related activities, primarily fossil fuel combustion, accounted for the vast majority of U.S. CO2 emissions for
the period of 1990 through 2015. Emissions from fossil fuel combustion comprise the vast majority of energy-
related emissions, with CO2 being the primary gas emitted (see Figure 2-5). Due to their relative importance, fossil
fuel combustion-related CO2 emissions are considered in detail in the Energy chapter (see Figure 2-6). In 2015,
2-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2015
approximately 82 percent of the energy consumed in the United States (on a Btu basis) was produced through the
combustion of fossil fuels. The remaining 18 percent came from other energy sources such as hydropower, biomass,
nuclear, wind, and solar energy. A discussion of specific trends related to CO2 as well as other greenhouse gas
emissions from energy consumption is presented in the Energy chapter. Energy-related activities are also responsible
for CH4 and N2O emissions (42 percent and 12 percent of total U.S. emissions of each gas, respectively). Table 2-4
presents greenhouse gas emissions from the Energy chapter, by source and gas.
Figure 2-5: 2015 Energy Chapter Greenhouse Gas Sources (MMT CO2 Eq.)
Trends 2-11
Figure 2-6: 2015 U.S. Fossil Carbon Flows (MMT CO2 Eq.)
Table 2-4: Emissions from Energy (MMT CO2 Eq.)
Gas/Source 1990 2005 2011 2012 2013 2014 2015
CO2 4,907.2 5,932.3 5,387.2 5,180.9 5,332.7 5,377.8 5,231.9
Mobile Combustiona 41.2 35.7 22.8 20.4 18.5 16.6 15.1
Incineration of Waste 0.5 0.4 0.3 0.3 0.3 0.3 0.3
2-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2015
International Bunker Fuelsc 0.9 1.0 1.0 0.9 0.9 0.9 0.9
Total 5,328.1 6,275.3 5,721.2 5,507.0 5,659.1 5,704.9 5,549.1
+ Does not exceed 0.05 MMT CO2 Eq. a There was a method update in this Inventory for estimating the share of gasoline used in on-road and non-road applications.
The change does not impact total U.S. gasoline consumption. It mainly results in a shift in gasoline consumption from the
transportation sector to industrial and commercial sectors for 2015, creating a break in the time series. The change is discussed
further in the Planned Improvements section of Chapter 3.1.
b Emissions from Wood Biomass and Biofuel Consumption are not included specifically in summing energy sector totals. Net
carbon fluxes from changes in biogenic carbon reservoirs are accounted for in the estimates for LULUCF. c Emissions from International Bunker Fuels are not included in totals.
Note: Totals may not sum due to independent rounding.
Carbon dioxide emissions from fossil fuel combustion are presented in Table 2-5 based on the underlying U.S.
energy consumer data collected by the U.S. Energy Information Administration (EIA). Estimates of CO2 emissions
from fossil fuel combustion are calculated from these EIA “end-use sectors” based on total consumption and
appropriate fuel properties (any additional analysis and refinement of the EIA data is further explained in the Energy
chapter of this report). EIA’s fuel consumption data for the electric power sector are comprised of electricity-only
and combined-heat-and-power (CHP) plants within the North American Industry Classification System (NAICS) 22
category whose primary business is to sell electricity, or electricity and heat, to the public (nonutility power
producers can be included in this sector as long as they meet they electric power sector definition). EIA statistics for
the industrial sector include fossil fuel consumption that occurs in the fields of manufacturing, agriculture, mining,
and construction. EIA’s fuel consumption data for the transportation sector consists of all vehicles whose primary
purpose is transporting people and/or goods from one physical location to another. EIA’s fuel consumption data for
the industrial sector consists of all facilities and equipment used for producing, processing, or assembling goods
(EIA includes generators that produce electricity and/or useful thermal output primarily to support on-site industrial
activities in this sector). EIA’s fuel consumption data for the residential sector consist of living quarters for private
households. EIA’s fuel consumption data for the commercial sector consist of service-providing facilities and
equipment from private and public organizations and businesses (EIA includes generators that produce electricity
and/or useful thermal output primarily to support the activities at commercial establishments in this sector). Table
2-5 and Figure 2-7 summarize CO2 emissions from fossil fuel combustion by end-use sector. Figure 2-8 further
describes the total emissions from fossil fuel combustion, separated by end-use sector, including CH4 and N2O in
addition to CO2.
Table 2-5: CO2 Emissions from Fossil Fuel Combustion by End-Use Sector (MMT CO2 Eq.)
Note: Totals may not sum due to independent rounding.
Some significant trends in U.S. emissions from Agriculture source categories include the following:
• Agricultural soils produced approximately 75.1 percent of N2O emissions in the United States in 2015.
Estimated emissions from this source in 2015 were 251.3 MMT CO2 Eq. Annual N2O emissions from
agricultural soils fluctuated between 1990 and 2015, although overall emissions were 2.0 percent lower in
2015 than in 1990. Year-to-year fluctuations are largely a reflection of annual variation in weather patterns,
synthetic fertilizer use, and crop production.
• Enteric fermentation is the largest anthropogenic source of CH4 emissions in the United States. In 2015,
enteric fermentation CH4 emissions were 166.5 MMT CO2 Eq. (25.4 percent of total CH4 emissions),
which represents an increase of 2.4 MMT CO2 Eq. (1.5 percent) since 1990. This increase in emissions
from 1990 to 2015 in enteric fermentation generally follows the increasing trends in cattle populations.
From 1990 to 1995, emissions increased and then generally decreased from 1996 to 2004, mainly due to
fluctuations in beef cattle populations and increased digestibility of feed for feedlot cattle. Emissions
increased from 2005 to 2007, as both dairy and beef populations increased. Research indicates that the feed
digestibility of dairy cow diets decreased during this period. Emissions decreased again from 2008 to 2015
as beef cattle populations again decreased.
• Liming and urea fertilization are the only source of CO2 emissions reported in the Agriculture sector.
Estimated emissions from these sources were 3.8 and 5.0 MMT CO2 Eq., respectively. Liming and urea
fertilization emissions increased by 5.6 percent and 5.3 percent, respectively, relative to 2014, and
decreased by 18.4 percent and increased by 108.2 percent, respectively since 1990.
• Overall, emissions from manure management increased 64.2 percent between 1990 and 2015. This
encompassed an increase of 78.3 percent for CH4, from 37.2 MMT CO2 Eq. in 1990 to 66.3 MMT CO2 Eq.
in 2015; and an increase of 26.6 percent for N2O, from 14.0 MMT CO2 Eq. in 1990 to 17.7 MMT CO2 Eq.
in 2015. The majority of the increase observed in CH4 resulted from swine and dairy cattle manure, where
emissions increased 58 and 136 percent, respectively, from 1990 to 2015. From 2014 to 2015, there was a
5.4 percent increase in total CH4 emissions from manure management, mainly due to minor shifts in the
animal populations and the resultant effects on manure management system allocations.
Land Use, Land-Use Change, and Forestry When humans alter the terrestrial biosphere through land use, changes in land use, and land management practices,
they also influence the carbon (C) stock fluxes on these lands and cause emissions of CH4 and N2O. Overall,
managed land is a net sink for CO2 (C sequestration) in the United States. The drivers of fluxes on managed lands
include, for example, forest management practices, tree planting in urban areas, the management of agricultural
2-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2015
soils, the landfilling of yard trimmings and food scraps, and activities that cause changes in C stocks in coastal
wetlands. The main drivers for net forest sequestration include net forest growth, increasing forest area, and a net
accumulation of C stocks in harvested wood pools. The net sequestration in Settlements Remaining Settlements, is
driven primarily by C stock gains in urban forests through net tree growth and increased urban area, as well as long-
term accumulation of C in landfills from additions of yard trimmings and food scraps.
The LULUCF sector in 2015 resulted in a net increase in C stocks (i.e., net CO2 removals) of 778.7 MMT CO2 Eq.
(Table 2-3).1 This represents an offset of approximately 11.8 percent of total (i.e., gross) greenhouse gas emissions
in 2015. Emissions of CH4 and N2O from LULUCF activities in 2015 were 19.7 MMT CO2 Eq. and represent 0.3
percent of total greenhouse gas emissions.2 Between 1990 and 2015, total C sequestration in the LULUCF sector
decreased by 6.2 percent, primarily due to a decrease in the rate of net C accumulation in forests and an increase in
CO2 emissions from Land Converted to Settlements.
Carbon dioxide removals from C stock changes are presented in Table 2-8 along with CH4 and N2O emissions for
LULUCF source categories. Forest fires were the largest source of CH4 emissions from LULUCF in 2015, totaling
7.3 MMT CO2 Eq. (292 kt of CH4). Coastal Wetlands Remaining Coastal Wetlands resulted in CH4 emissions of 3.6
MMT CO2 Eq. (143 kt of CH4). Grassland fires resulted in CH4 emissions of 0.4 MMT CO2 Eq. (16 kt of CH4).
Peatlands Remaining Peatlands, Land Converted to Wetlands, and Drained Organic Soils resulted in CH4 emissions
of less than 0.05 MMT CO2 Eq. each.
Forest fires were also the largest source of N2O emissions from LULUCF in 2015, totaling 4.8 MMT CO2 Eq. (16 kt
of N2O). Nitrous oxide emissions from fertilizer application to settlement soils in 2015 totaled to 2.5 MMT CO2 Eq.
(8 kt of N2O). This represents an increase of 76.6 percent since 1990. Additionally, the application of synthetic
fertilizers to forest soils in 2015 resulted in N2O emissions of 0.5 MMT CO2 Eq. (2 kt of N2O). Nitrous oxide
emissions from fertilizer application to forest soils have increased by 455 percent since 1990, but still account for a
relatively small portion of overall emissions. Grassland fires resulted in N2O emissions of 0.4 MMT CO2 Eq. (1 kt
of N2O). Coastal Wetlands Remaining Coastal Wetlands and Drained Organic Soils resulted in N2O emissions of
0.1 MMT CO2 Eq. each (less than 0.5 kt of N2O), and Peatlands Remaining Peatlands resulted in N2O emissions of
less than 0.05 MMT CO2 Eq. (see Table 2-8).
Table 2-8: U.S. Greenhouse Gas Emissions and Removals (Net Flux) from Land Use, Land-Use Change, and Forestry (MMT CO2 Eq.)
1 LULUCF Carbon Stock Change is the net C stock change from the following categories: Forest Land Remaining Forest Land,
Land Converted to Forest Land, Cropland Remaining Cropland, Land Converted to Cropland, Grassland Remaining Grassland,
Land Converted to Grassland, Wetlands Remaining Wetlands, Land Converted to Wetlands, Settlements Remaining Settlements,
and Land Converted to Settlements. 2 LULUCF emissions include the CH4 and N2O emissions reported for Peatlands Remaining Peatlands, Forest Fires, Drained
Organic Soils, Grassland Fires, and Coastal Wetlands Remaining Coastal Wetlands; CH4 emissions from Land Converted to
Coastal Wetlands; and N2O emissions from Forest Soils and Settlement Soils.
LULUCF Sector Net Totale (819.6) (731.0) (749.2) (753.8) (763.0) (761.4) (758.9)
+ Absolute value does not exceed 0.05 MMT CO2 Eq. a LULUCF Carbon Stock Change is the net C stock change from the following categories: Forest Land Remaining Forest
Land, Land Converted to Forest Land, Cropland Remaining Cropland, Land Converted to Cropland, Grassland
Remaining Grassland, Land Converted to Grassland, Wetlands Remaining Wetlands, Land Converted to Wetlands,
Settlements Remaining Settlements, and Land Converted to Settlements. b Estimates include emissions from N fertilizer additions on both Settlements Remaining Settlements and Land Converted to
Settlements. c Estimates include emissions from N fertilizer additions on both Forest Land Remaining Forest Land and Land Converted
to Forest Land. d LULUCF emissions include the CH4 and N2O emissions reported for Peatlands Remaining Peatlands, Forest Fires,
Drained Organic Soils, Grassland Fires, and Coastal Wetlands Remaining Coastal Wetlands; CH4 emissions from Land
Converted to Coastal Wetlands; and N2O emissions from Forest Soils and Settlement Soils. e The LULUCF Sector Net Total is the net sum of all CH4 and N2O emissions to the atmosphere plus net carbon stock
changes.
Notes: Totals may not sum due to independent rounding. Parentheses indicate net sequestration.
Other significant trends from 1990 to 2015 in emissions from LULUCF categories include:
• Annual C sequestration by forest land (i.e., annual C stock accumulation in the five C pools and harvested
wood products for Forest Land Remaining Forest Land and Land Converted to Forest Land) has decreased
by approximately 6.1 percent since 1990. This is primarily due to decreased C stock gains in Land
Converted to Forest Land and the harvested wood products pools within Forest Land Remaining Forest
Land.
• Annual C sequestration from Settlements Remaining Settlements (which includes organic soils, urban trees,
and landfilled yard trimmings and food scraps) has increased by 18.4 percent over the period from 1990 to
2015. This is primarily due to an increase in urbanized land area in the United States.
• Annual emissions from Land Converted to Grassland increased by approximately 14.4 percent from 1990
to 2015 due to losses in aboveground biomass, belowground biomass, dead wood, and litter C stocks from
Forest Land Converted to Grassland.
• Annual emissions from Land Converted to Settlements increased by approximately 83.5 percent from 1990
to 2015 due to losses in aboveground biomass C stocks from Forest Land Converted to Settlements and
mineral soils C stocks from Grassland Converted to Settlements.
2-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2015
Waste Waste management and treatment activities are sources of greenhouse gas emissions (see Figure 2-12). In 2015,
landfills were the third-largest source of U.S. anthropogenic CH4 emissions, accounting for 17.6 percent of total
U.S. CH4 emissions.3 Additionally, wastewater treatment accounts for 14.2 percent of Waste emissions, 2.3 percent
of U.S. CH4 emissions, and 1.5 percent of N2O emissions. Emissions of CH4 and N2O from composting grew from
1990 to 2015, and resulted in emissions of 4.0 MMT CO2 Eq. in 2015. A summary of greenhouse gas emissions
from the Waste chapter is presented in Table 2-9.
Figure 2-12: 2015 Waste Chapter Greenhouse Gas Sources (MMT CO2 Eq.)
Overall, in 2015, waste activities generated emissions of 139.4 MMT CO2 Eq., or 2.1 percent of total U.S.
Notes: Total emissions presented without LULUCF. Total net emissions presented with LULUCF.
+ Does not exceed 0.05 MMT CO2 Eq. or 0.05 percent. a There was a method update in this Inventory for estimating the share of gasoline used in on-road and non-road applications.
The change does not impact total U.S. gasoline consumption. It mainly results in a shift in gasoline consumption from the
transportation sector to industrial and commercial sectors for 2015, creating a break in the time series. The change is
discussed further in the Planned Improvements section of Chapter 3.1. b Percent of total (gross) emissions excluding emissions from LULUCF for 2015. c The LULUCF Sector Net Total is the net sum of all CH4 and N2O emissions to the atmosphere plus net carbon stock
changes.
Notes: Totals may not sum due to independent rounding. Parentheses indicate negative values or sequestration.
Emissions with Electricity Distributed to Economic Sectors It can also be useful to view greenhouse gas emissions from economic sectors with emissions related to electricity
generation distributed into end-use categories (i.e., emissions from electricity generation are allocated to the
economic sectors in which the electricity is consumed). The generation, transmission, and distribution of electricity,
which is the largest economic sector in the United States, accounted for 29 percent of total U.S. greenhouse gas
emissions in 2015. Emissions increased by 4 percent since 1990, as electricity demand grew and fossil fuels
remained the dominant energy source for generation. Electricity generation-related emissions decreased from 2014
to 2015 by 6.7 percent, primarily due to decreased CO2 emissions from fossil fuel combustion due to an increase in
natural gas consumption, and decreased coal consumption. Electricity sales to the residential and commercial end-
use sectors in 2015 decreased by 0.2 percent and increased by 0.6 percent, respectively. The trend in the residential
and commercial sectors can largely be attributed to warmer, less energy-intensive winter conditions compared to
2014. Electricity sales to the industrial sector in 2015 decreased by approximately 1.1 percent. Overall, in 2015, the
amount of electricity generated (in kWh) decreased by 0.2 percent from the previous year. This decrease in
generation contributed to a reduction in CO2 emissions from the electric power sector of 6.7 percent, as the
consumption of CO2-intensive coal for electricity generation decreased by 13.9 percent and natural gas generation
increased by 18.7 percent. The consumption of petroleum for electricity generation decreased by 6.6 percent in 2015
relative to 2014. Table 2-11 provides a detailed summary of emissions from electricity generation-related activities.
Table 2-11: Electricity Generation-Related Greenhouse Gas Emissions (MMT CO2 Eq.)
Gas/Fuel Type or Source 1990 2005 2011 2012 2013 2014 2015 CO2 1,831.2 2,416.5 2,172.9 2,036.6 2,053.7 2,054.5 1,917.0
Total 1,862.5 2,441.6 2,197.3 2,059.9 2,078.2 2,079.7 1,941.4
+ Does not exceed 0.05 MMT CO2 Eq. a Includes only stationary combustion emissions related to the generation of electricity.
Note: Totals may not sum due to independent rounding.
To distribute electricity emissions among economic end-use sectors, emissions from the source categories assigned
to the electricity generation sector were allocated to the residential, commercial, industry, transportation, and
agriculture economic sectors according to each economic sector’s share of retail sales of electricity consumption
(EIA 2017 and Duffield 2006). These source categories include CO2 from Fossil Fuel Combustion, CH4 and N2O
from Stationary Combustion, Incineration of Waste, Other Process Uses of Carbonates, and SF6 from Electrical
Transmission and Distribution Systems. Note that only 50 percent of the Other Process Uses of Carbonates
emissions were associated with electricity generation and distributed as described; the remainder of Other Process
Uses of Carbonates emissions were attributed to the industrial processes economic end-use sector.4
When emissions from electricity are distributed among these sectors, industrial activities account for the largest
share of total U.S. greenhouse gas emissions (29.3 percent), followed closely by emissions from transportation (27.5
percent). Emissions from the residential and commercial sectors also increase substantially when emissions from
electricity are included. In all sectors except agriculture, CO2 accounts for more than 82 percent of greenhouse gas
emissions, primarily from the combustion of fossil fuels.
Table 2-12 presents a detailed breakdown of emissions from each of these economic sectors, with emissions from
electricity generation distributed to them. Figure 2-14 shows the trend in these emissions by sector from 1990 to
2015.
Figure 2-14: U.S. Greenhouse Gas Emissions with Electricity-Related Emissions Distributed to Economic Sectors (MMT CO2 Eq.)
4 Emissions were not distributed to U.S. Territories, since the electricity generation sector only includes emissions related to the
generation of electricity in the 50 states and the District of Columbia.
2-28 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2015
Table 2-12: U.S. Greenhouse Gas Emissions by Economic Sector and Gas with Electricity-Related Emissions Distributed (MMT CO2 Eq.) and Percent of Total in 2015
Notes: Total emissions presented without LULUCF. Net emissions presented with LULUCF.
+ Does not exceed 0.05 MMT CO2 Eq. or 0.05 percent. a There was a method update in this Inventory for estimating the share of gasoline used in on-road and non-road
applications. The change does not impact total U.S. gasoline consumption. It mainly results in a shift in gasoline
consumption from the transportation sector to industrial and commercial sectors for 2015, creating a break in the time
series. The change is discussed further in the Planned Improvements section of Chapter 3.1. b Percent of total gross emissions excluding emissions from LULUCF for year 2015. c Includes primarily HFC-134a. d The LULUCF Sector Net Total is the net sum of all CH4 and N2O emissions to the atmosphere plus net carbon stock
changes.
Notes: Emissions from electricity generation are allocated based on aggregate electricity consumption in each end-use
sector. Totals may not sum due to independent rounding.
Industry The industry end-use sector includes CO2 emissions from fossil fuel combustion from all manufacturing facilities, in
aggregate. This end-use sector also includes emissions that are produced as a byproduct of the non-energy-related
industrial process activities. The variety of activities producing these non-energy-related emissions includes CH4
emissions from petroleum and natural gas systems, fugitive CH4 emissions from coal mining, by-product CO2
emissions from cement manufacture, and HFC, PFC, SF6, and NF3 byproduct emissions from semiconductor
manufacture, to name a few. Since 1990, industrial sector emissions have declined. The decline has occurred both in
direct emissions and indirect emissions associated with electricity use. In theory, emissions from the industrial end-
use sector should be highly correlated with economic growth and industrial output, but heating of industrial
buildings and agricultural energy consumption are also affected by weather conditions. In addition, structural
changes within the U.S. economy that lead to shifts in industrial output away from energy-intensive manufacturing
products to less energy-intensive products (e.g., from steel to computer equipment) also have a significant effect on
industrial emissions.
Transportation When electricity-related emissions are distributed to economic end-use sectors, transportation activities accounted
for 27.5 percent of U.S. greenhouse gas emissions in 2015. The largest sources of transportation greenhouse gases in
2015 were passenger cars (41.9 percent), freight trucks (22.9 percent), light-duty trucks, which include sport utility
vehicles, pickup trucks, and minivans (18.0 percent), commercial aircraft (6.6 percent), rail (2.6 percent), other
aircraft (2.2 percent), pipelines (2.1 percent), and ships and boats (1.8 percent). These figures include direct CO2,
CH4, and N2O emissions from fossil fuel combustion used in transportation and emissions from non-energy use (i.e.,
lubricants) used in transportation, as well as HFC emissions from mobile air conditioners and refrigerated transport
allocated to these vehicle types.
In terms of the overall trend, from 1990 to 2015, total transportation emissions increased due, in large part, to
increased demand for travel. The number of vehicle miles traveled (VMT) by light-duty motor vehicles (passenger
cars and light-duty trucks) increased 40 percent from 1990 to 2015,5 as a result of a confluence of factors including
population growth, economic growth, urban sprawl, and periods of low fuel prices. The decline in new light-duty
vehicle fuel economy between 1990 and 2004 reflected the increasing market share of light-duty trucks, which grew
5 VMT estimates are based on data from FHWA Highway Statistics Table VM-1 (FHWA 1996 through 2016). In 2011, FHWA
changed its methods for estimating VMT by vehicle class, which led to a shift in VMT and emissions among on-road vehicle
classes in the 2007 to 2015 time period. In absence of these method changes, light-duty VMT growth between 1990 and 2015
would likely have been even higher.
2-30 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2015
from about 30 percent of new vehicle sales in 1990 to 48 percent in 2004. Starting in 2005, average new vehicle fuel
economy began to increase while light-duty VMT grew only modestly for much of the period. Light-duty VMT
grew by less than one percent or declined each year between 2005 and 20136 and has since grown a faster rate (1.2
percent from 2013 to 2014, and 2.6 percent from 2014 to 2015). Average new vehicle fuel economy has improved
almost every year since 2005 and the truck share decreased to about 33 percent in 2009, and has since varied from
year to year between 36 percent and 43 percent. Truck share is about 43 percent of new vehicles in model year 2015
(EPA 2016a). Table 2-13 provides a detailed summary of greenhouse gas emissions from transportation-related
activities with electricity-related emissions included in the totals. It is important to note that there was a change in
methods between 2014 and 2015 used to estimate gasoline consumption in the transportation sector. In the absence
of this change, CO2 emissions from passenger cars, light-duty trucks, and other on-road vehicles using gasoline
would likely have been higher in 2015.7
Almost all of the energy consumed for transportation was supplied by petroleum-based products, with more than
half being related to gasoline consumption in automobiles and other highway vehicles. Other fuel uses, especially
diesel fuel for freight trucks and jet fuel for aircraft, accounted for the remainder. The primary driver of
transportation-related emissions was CO2 from fossil fuel combustion, which increased by 16 percent from 1990 to
2015.8 This rise in CO2 emissions, combined with an increase in HFCs from close to zero emissions in 1990 to 45.1
MMT CO2 Eq. in 2015, led to an increase in overall emissions from transportation activities of 16 percent.9
Table 2-13: Transportation-Related Greenhouse Gas Emissions (MMT CO2 Eq.)
6 In 2007 and 2008 light-duty VMT decreased 3 percent and 2.3 percent, respectively. Note that the decline in light-duty VMT
from 2006 to 2007 is due at least in part to a change in FHWA's methods for estimating VMT. In absence of these method
changes, light-duty VMT growth between 2006 and 2007 would likely have been higher. See previous footnote. 7 There was a method update in this Inventory for estimating the share of gasoline used in on-road and non-road applications.
The change does not impact total U.S. gasoline consumption. It mainly results in a shift in gasoline consumption from the
transportation sector to industrial and commercial sectors for 2015, creating a break in the time series. The change is discussed
further in the Planned Improvements section of Chapter 3.1. 8 See previous footnote. 9 See previous footnote.
Fuelsg 104.5 114.2 112.8 106.8 100.7 104.2 111.8 Ethanol CO2
h 4.1 22.4 71.5 71.5 73.4 74.9 75.9
Biodiesel CO2h 0.0 0.9 8.3 8.5 13.5 13.3 14.1
+ Does not exceed 0.05 MMT CO2 Eq. a There was a method update in this Inventory for estimating the share of gasoline used in on-road and
non-road applications. The change does not impact total U.S. gasoline consumption. It mainly results in a
shift in gasoline consumption from the transportation sector to industrial and commercial sectors for
2015, creating a break in the time series. The change is discussed further in the Planned Improvements
section of Chapter 3.1. b Consists of emissions from jet fuel consumed by domestic operations of commercial aircraft (no
bunkers). c Consists of emissions from jet fuel and aviation gasoline consumption by general aviation and military
aircraft. d Fluctuations in emission estimates are associated with fluctuations in reported fuel consumption, and
may reflect issues with data sources. e Other emissions from electricity generation are a result of waste incineration (as the majority of
municipal solid waste is combusted in “trash-to-steam” electricity generation plants), electrical
transmission and distribution, and a portion of Other Process Uses of Carbonates (from pollution control
equipment installed in electricity generation plants). f CO2 estimates reflect natural gas used to power pipelines, but not electricity. While the operation of
pipelines produces CH4 and N2O, these emissions are not directly attributed to pipelines in the U.S.
Inventory. g Emissions from International Bunker Fuels include emissions from both civilian and military activities;
these emissions are not included in the transportation totals. h Ethanol and biodiesel CO2 estimates are presented for informational purposes only. See Section 3.10 and
the estimates in Land Use, Land-Use Change, and Forestry (see Chapter 6), in line with IPCC
methodological guidance and UNFCCC reporting obligations, for more information on ethanol and
biodiesel.
Notes: Passenger cars and light-duty trucks include vehicles typically used for personal travel and less
than 8,500 lbs; medium- and heavy-duty trucks include vehicles larger than 8,500 lbs. HFC emissions
primarily reflect HFC-134a. Totals may not sum due to independent rounding.
2-32 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2015
Commercial The commercial sector is heavily reliant on electricity for meeting energy needs, with electricity consumption for
lighting, heating, air conditioning, and operating appliances. The remaining emissions were largely due to the direct
consumption of natural gas and petroleum products, primarily for heating and cooking needs. Energy-related
emissions from the residential and commercial sectors have generally been increasing since 1990, and are often
correlated with short-term fluctuations in energy consumption caused by weather conditions, rather than prevailing
economic conditions. Landfills and wastewater treatment are included in this sector, with landfill emissions
decreasing since 1990 and wastewater treatment emissions decreasing slightly.
Residential The residential sector is heavily reliant on electricity for meeting energy needs, with electricity consumption for
lighting, heating, air conditioning, and operating appliances. The remaining emissions were largely due to the direct
consumption of natural gas and petroleum products, primarily for heating and cooking needs. Emissions from the
residential sectors have generally been increasing since 1990, and are often correlated with short-term fluctuations in
energy consumption caused by weather conditions, rather than prevailing economic conditions. In the long-term, this
sector is also affected by population growth, regional migration trends, and changes in housing and building
attributes (e.g., size and insulation).
Agriculture The agriculture end-use sector includes a variety of processes, including enteric fermentation in domestic livestock,
livestock manure management, and agricultural soil management. In 2015, agricultural soil management was the
largest source of N2O emissions, and enteric fermentation was the largest source of CH4 emissions in the United
States. This sector also includes small amounts of CO2 emissions from fossil fuel combustion by motorized farm
equipment like tractors. The agriculture sector is less reliant on electricity than the other sectors.
Box 2-1: Methodology for Aggregating Emissions by Economic Sector
In presenting the Economic Sectors in the annual Inventory of U.S. Greenhouse Gas Emissions and Sinks, the
Inventory expands upon the standard IPCC sectors common for UNFCCC reporting. Discussing greenhouse gas
emissions relevant to U.S.-specific sectors improves communication of the report’s findings.
In the Electricity Generation economic sector, CO2 emissions from the combustion of fossil fuels included in the
EIA electric utility fuel consuming sector are apportioned to this economic sector. Stationary combustion emissions
of CH4 and N2O are also based on the EIA electric utility sector. Additional sources include CO2, CH4, and N2O
from waste incineration, as the majority of municipal solid waste is combusted in “trash-to-steam” electricity
generation plants. The Electricity Generation economic sector also includes SF6 from Electrical Transmission and
Distribution, and a portion of CO2 from Other Process Uses of Carbonates (from pollution control equipment
installed in electricity generation plants).
In the Transportation economic sector, the CO2 emissions from the combustion of fossil fuels included in the EIA
transportation fuel consuming sector are apportioned to this economic sector (additional analyses and refinement of
the EIA data is further explained in the Energy chapter of this report). Emissions of CH4 and N2O from Mobile
Combustion are also apportioned to this economic sector based on the EIA transportation fuel consuming sector.
Substitution of Ozone Depleting Substances emissions are apportioned based on their specific end-uses within the
source category, with emissions from transportation refrigeration/air-conditioning systems to this economic sector.
Finally, CO2 emissions from Non-Energy Uses of Fossil Fuels identified as lubricants for transportation vehicles are
included in the Transportation economic sector.
For the Industry economic sector, the CO2 emissions from the combustion of fossil fuels included in the EIA
industrial fuel consuming sector, minus the agricultural use of fuel explained below, are apportioned to this
economic sector. The CH4 and N2O emissions from stationary and mobile combustion are also apportioned to this
economic sector based on the EIA industrial fuel consuming sector, minus emissions apportioned to the Agriculture
Trends 2-33
economic sector described below. Substitution of Ozone Depleting Substances emissions are apportioned based on
their specific end-uses within the source category, with most emissions falling within the Industry economic sector.
Additionally, all process-related emissions from sources with methods considered within the IPCC IPPU sector have
been apportioned to this economic sector. This includes the process-related emissions (i.e., emissions from the actual
process to make the material, not from fuels to power the plant) from such activities as Cement Production, Iron and
Steel Production and Metallurgical Coke Production, and Ammonia Production. Additionally, fugitive emissions
from energy production sources, such as Natural Gas Systems, Coal Mining, and Petroleum Systems are included in
the Industry economic sector. A portion of CO2 from Other Process Uses of Carbonates (from pollution control
equipment installed in large industrial facilities) are also included in the Industry economic sector. Finally, all
remaining CO2 emissions from Non-Energy Uses of Fossil Fuels are assumed to be industrial in nature (besides the
lubricants for transportation vehicles specified above), and are attributed to the Industry economic sector.
As agriculture equipment is included in EIA’s industrial fuel consuming sector surveys, additional data is used to
extract the fuel used by agricultural equipment, to allow for accurate reporting in the Agriculture economic sector
from all sources of emissions, such as motorized farming equipment. Energy consumption estimates are obtained
from Department of Agriculture survey data, in combination with separate EIA fuel sales reports. This
supplementary data is used to apportion some of the CO2 emissions from fossil fuel combustion, and CH4 and N2O
emissions from stationary and mobile combustion, to the Agriculture economic sector. The other emission sources
included in this economic sector are intuitive for the agriculture sectors, such as N2O emissions from Agricultural
Soils, CH4 from Enteric Fermentation, CH4 and N2O from Manure Management, CH4 from Rice Cultivation, CO2
emissions from Liming and Urea Application, and CH4 and N2O from Forest Fires. Nitrous oxide emissions from
the Application of Fertilizers to tree plantations (termed “forest land” by the IPCC) are also included in the
Agriculture economic sector.
The Residential economic sector includes the CO2 emissions from the combustion of fossil fuels reported for the
EIA residential sector. Stationary combustion emissions of CH4 and N2O are also based on the EIA residential fuel
consuming sector. Substitution of Ozone Depleting Substances are apportioned based on their specific end-uses
within the source category, with emissions from residential air-conditioning systems to this economic sector. Nitrous
oxide emissions from the Application of Fertilizers to developed land (termed “settlements” by the IPCC) are also
included in the Residential economic sector.
The Commercial economic sector includes the CO2 emissions from the combustion of fossil fuels reported in the
EIA commercial fuel consuming sector data. Emissions of CH4 and N2O from Mobile Combustion are also
apportioned to this economic sector based on the EIA transportation fuel consuming sector. Substitution of Ozone
Depleting Substances emissions are apportioned based on their specific end-uses within the source category, with
emissions from commercial refrigeration/air-conditioning systems apportioned to this economic sector. Public works
sources including direct CH4 from Landfills and CH4 and N2O from Wastewater Treatment and Composting are also
included in this economic sector.
Box 2-2: Recent Trends in Various U.S. Greenhouse Gas Emissions-Related Data
Total emissions can be compared to other economic and social indices to highlight changes over time. These
comparisons include: (1) emissions per unit of aggregate energy consumption, because energy-related activities are
the largest sources of emissions; (2) emissions per unit of fossil fuel consumption, because almost all energy-related
emissions involve the combustion of fossil fuels; (3) emissions per unit of electricity consumption, because the
electric power industry—utilities and non-utilities combined—was the largest source of U.S. greenhouse gas
emissions in 2015; (4) emissions per unit of total gross domestic product as a measure of national economic activity;
or (5) emissions per capita.
Table 2-14 provides data on various statistics related to U.S. greenhouse gas emissions normalized to 1990 as a
baseline year. These values represent the relative change in each statistic since 1990. Greenhouse gas emissions in
the United States have grown at an average annual rate of 0.2 percent since 1990. Since 1990, this rate is slightly
slower than that for total energy and for fossil fuel consumption, and much slower than that for electricity
consumption, overall gross domestic product (GDP) and national population (see Table 2-14 and Figure 2-15).
These trends vary relative to 2005, when greenhouse gas emissions, total energy and fossil fuel consumption began
to peak. Greenhouse gas emissions in the United States have decreased at an average annual rate of 1.0 percent since
2-34 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2015
2005. Total energy and fossil fuel consumption have also decreased at slower rates than emissions since 2005, while
electricity consumption, GDP, and national population continued to increase.
Table 2-14: Recent Trends in Various U.S. Data (Index 1990 = 100)
a Average annual growth rate b GWP-weighted values c Energy-content-weighted values (EIA 2017) d Gross Domestic Product in chained 2009 dollars (BEA 2017) e U.S. Census Bureau (2016)
Figure 2-15: U.S. Greenhouse Gas Emissions Per Capita and Per Dollar of Gross Domestic Product
Source: BEA (2017), U.S. Census Bureau (2016), and emission estimates in this report.
Trends 2-35
2.3 Indirect Greenhouse Gas Emissions (CO, NOx, NMVOCs, and SO2)
The reporting requirements of the UNFCCC10 request that information be provided on indirect greenhouse gases,
which include CO, NOx, NMVOCs, and SO2. These gases do not have a direct global warming effect, but indirectly
affect terrestrial radiation absorption by influencing the formation and destruction of tropospheric and stratospheric
ozone, or, in the case of SO2, by affecting the absorptive characteristics of the atmosphere. Additionally, some of
these gases may react with other chemical compounds in the atmosphere to form compounds that are greenhouse
gases. Carbon monoxide is produced when carbon-containing fuels are combusted incompletely. Nitrogen oxides
(i.e., NO and NO2) are created by lightning, fires, fossil fuel combustion, and in the stratosphere from N2O. Non-
methane volatile organic compounds—which include hundreds of organic compounds that participate in
atmospheric chemical reactions (i.e., propane, butane, xylene, toluene, ethane, and many others)—are emitted
primarily from transportation, industrial processes, and non-industrial consumption of organic solvents. In the
United States, SO2 is primarily emitted from coal combustion for electric power generation and the metals industry.
Sulfur-containing compounds emitted into the atmosphere tend to exert a negative radiative forcing (i.e., cooling)
and therefore are discussed separately.
One important indirect climate change effect of NMVOCs and NOx is their role as precursors for tropospheric ozone
formation. They can also alter the atmospheric lifetimes of other greenhouse gases. Another example of indirect
greenhouse gas formation into greenhouse gases is the interaction of CO with the hydroxyl radical—the major
atmospheric sink for CH4 emissions—to form CO2. Therefore, increased atmospheric concentrations of CO limit the
number of hydroxyl molecules (OH) available to destroy CH4.
Since 1970, the United States has published estimates of emissions of CO, NOx, NMVOCs, and SO2 (EPA 2015),11
which are regulated under the Clean Air Act. Table 2-15 shows that fuel combustion accounts for the majority of
emissions of these indirect greenhouse gases. Industrial processes—such as the manufacture of chemical and allied
products, metals processing, and industrial uses of solvents—are also significant sources of CO, NOx, and
NMVOCs.
Table 2-15: Emissions of NOx, CO, NMVOCs, and SO2 (kt)
Industrial Processes and Product Use 4,129 1,557 1,229 1,246 1,262 1,273 1,273
Oil and Gas Activities 302 318 610 666 723 780 780
10 See <http://unfccc.int/resource/docs/2013/cop19/eng/10a03.pdf>. 11 NOx and CO emission estimates from Field Burning of Agricultural Residues were estimated separately, and therefore not
taken from EPA (2016b).
2-36 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990–2015